1. Field of the Invention
The present invention relates to an air-cooled electric motor, in particular, for a hand-held power tool, and having an air duct.
2. Description of the Prior Art
The stator of an electric motor is formed of stacked sheet metal lamellas in order to prevent any eddy current losses in an a.c. field. The necessary discrete number of lamellas results, upon assembly, to a discrete lengths, with a length tolerance of about 0.5 mm with a minimum one lamella.
For removal of heat, which is generated during operation of the electric motor, in compact electrical motors which are preferably used in hand-held power tools, a cooling air stream is directed through a gap between the stator and the rotor. The air stream is generated by a fan impeller mounted on the rotor shaft. In order to increase the efficiency, the fan impeller is movable in a suitably shaped, air duct with a shroud, whereby a critical, axially narrow gap for the fan impeller is maintained. Even smaller changes of +/−0.2 mm with a slot having a width of about 1 mm cause strong fluctuations of +/−50% of the generated air flow, which leads to different cooling characteristics within a tool series.
According to German Publication DE 42 20 078 A1, in front of the fan impeller, an annular air duct, which forms a foreground grid, is secured to the tool housing of a hand-held tool. The securing of a stator with the air duct cannot be carried out.
In U.S. Pat. No. 5,343,101, behind the fan impeller, an annular or ring-shaped air duct, which is formed of a plastic material, is secured to a rotor end shield with screws. The rotor end shield is secured in the tool housing and is provided with bushes each having an inner thread for a locking bolt for securing the stator. U.S. Pat. No. 5,343,101 does not disclose a freely projecting shroud having a predetermined position relative to the impeller and arranged on one side of the impeller.
According to German Publication DE 26 17 860, a naturally stiff receiving ring, which functions as an annular air duct, is arranged in front of the impeller, with the receiving ring being secured to the tool housing and provided with bushes having an inner thread for looking bolts for securing the stator. Upon securing the stator using the rigid receiving ring, the stator axial tolerances result in the change of the critical axial width of the gap in front of the fan impeller.
Accordingly, an object of the present invention is an electric motor with an air duct which is arranged between the rotor and the fan impeller and which, in addition to functioning as means for securing the stator, provides for a gap in front of the impeller that does not depend from stator tolerances.